The present invention relates to memory arrays, and more particularly to a method and system for providing a polysilicon stringer monitor.
Achieving higher yields continues to be a desired goal during memory chip fabrication. The various processes and techniques used to manufacture chips have therefore become increasingly important. Part of the process involved in manufacturing a flash memory array, for example, requires creating an array of CMOS (Complementary MOS) transistors using layers of polysilicon.
The flash memory array includes columns of active regions that are separated by columns of insulating field oxide regions. The transistors are spaced apart in the active regions and each a row of transistors are bits in a memory word. The transistors are formed with various materials including a type-1 layer of polysilicon, and transistors forming a row in the array are connected by a word-line comprising a type-2 layer of polysilicon.
FIGS. 1-4 are diagrams illustrating the process of creating such a memory array. FIG. 1A is a top view of a portion of a substrate 10 and FIG. 1B is a cross-sectional view of the substrate 10 along line A. The first step of the process is creating the columns of active regions 12 and insulating field oxide regions (FOX) 14.
FIGS. 2A and 2B are top and cross-sectional views of the substrate 10, respectively, showing that a layer of tunnel oxide 16 is then deposited over the active regions 12 followed by the deposition of a type-1 layer of polysilicon 18, which is referred to as a poly1 layer. The poly1 layer 18 is then masked and etched as shown.
FIG. 3A is a top view of the substrate 10 showing that a layer of oxide nitride (ONO) 20 is then deposited over the poly1 layer 18, and that a type-2 layer of polysilicon 22 (poly2) is patterned over the ONO 20 to form word lines. FIG. 3B is a cross-sectional view of the substrate 10 along line B, and FIG. 3B is a cross-sectional view of the substrate 10 along line C.
In FIG. 4A, after the poly2 layer is deposited, an etch is performed on the ONO 20 and the poly1 layer 18 between the rows of the poly2 layer 22, which is also shown in the cross-sectional view of FIG. 4B.
FIG. 4C is a cross-sectional perspective view of the substrate along line E after the etching of the ONO 20 and the poly1 layer 18. The etching process is anisotropic, meaning that it removes material directionally to a predetermined depth. But due to the shape of the FOX regions 14, the thickness of the ONO 20 and the poly1 layer 18 at the vertical edges of the poly1 layer 18 is greater than their average thicknesses. Therefore, the etching process sometimes fails to remove all of the ONO 20 and the poly1 18 from the active regions 12 between the rows of poly2 22, leaving what is called a poly1 stringer 26. The presence of a poly1 stringer 26 provides a contact between the two adjacent transistors, which causes problems in the memory array.
Accordingly, what is needed is a system and method for detecting the presence of poly1 stringers on a memory array. The present invention addresses such a need.
The present invention provides a system and method for detecting the presence of poly1 stringers on a memory array. The polysilicon stringer monitor includes a continuous type-2 layer of polysilicon forming a first row and a second row across the active region and covering the active region in-between the first and second rows. The polysilicon stringer monitor further includes a continuous type-1 layer of polysilicon extending under the first row, wherein the type-1 layer also covers the active area in-between the first and second rows as well as covers the active area under the second row.
According to the present invention, extending the type-1 and type-2 layers of polysilicon in this manner effectively enables the detection of poly1 stringers in the active areas of the memory array.